Test Hyperledger Sawtooth events

[mhdirkse]

[AX-20] created by mdi

[mhdirkse]

by dirkse

[mhdirkse]

by dirkse

[mhdirkse]

Hyperledger Sawtooth can generate events that broadcast changes of the blockchain. This functionality allows users of the blockchain to provide search functionality. I need events to be able to navigate through the blockchain. Therefore I need to try them by programming a simple example.

[mhdirkse]

I understand Hyperledger Sawtooth event. There are code samples in Golang in the Sawtooth documentation. I included that in my "hello world" Sawtooth example.

I found the following:

• You can subscribe to block-commit events. A block commit event contains the id if the new block and the id of the previous block. This allows you to detect forks.
• I know how to let the processor emit custom events and I can subscribe to these with my eventListener executable.
• Subscribing to events and receiving events goes over the same ZMQ socket and happens between the same servers. Therefore, the sequence of messages is preserved. See https://lists.zeromq.org/pipermail/zeromq-dev/2015-January/027748.html
• When you subscribe both to block-commit events and to a custom event type, the block-commit events come first. This allows you to assign a block to a custom event.
• When you subscribe to events, you can request event catch-up by including a block id. Then events from next blocks are sent, not events from the given block id itself.
• The above is communication using ZMQ, directly to a validator.
• You can use the REST interface to get the id of the first block. Do a GET request to http::8008/blocks. The blocks are given in reverse order. The header_signature field gives the block id. Previous block ids are also included.

[mhdirkse]

Event messages have three elements:

• A type, which is a string. Typically it is prefixed with the family name that is returned by the handler. This is the handler added to the transaction processor.
• Attributes, which are name/value pairs. When you subscribe to events, you can filter on the attributes.
• A data buffer that is opaque to Hyperledger Sawtooth. The receiver of the event can parse it based on the event type.